Capacitive touch sensors generally include a sensor array configured as a matrix of sensor bars arranged in horizontal and vertical directions. Each sensor bar is coupled to a control circuit. The control circuit measures capacitive loading on the array to determine the position or location of the touch on the matrix. The control circuit measures capacitive loading by providing a drive signal to each sensor bar and receiving a sensor signal from each sensor bar. The control circuit analyzes the sensor signals to measure the capacitive loading on the matrix. The measurement of capacitive loading on the horizontal sensor bars allows the capacitive touch sensor to determine the vertical location of the touch, and the measurement of capacitive loading on the vertical sensor bars allows the capacitive touch sensor to determine the horizontal location of the touch.
Sensor arrays including sensor bars disposed in both vertical and horizontal directions have certain drawbacks. For example, a sensor array having a matrix of sensor bars generally requires a large number of layers which are expensive to manufacture. Also, each bar (or set of bars) requires separate sense, drive, and switching circuitry within the control circuit. Further, interleaving sensor bars in two directions (e.g., horizontal and vertical) increases the cost of the sensor array and detrimentally affects the optical performance of the display. Therefore, there is a need for a sensor array having a reduced number of sensor bars which does not utilize an interleaved matrix of sensor bars.
Conventional single sheet capacitive touch sensors are problematic because they are susceptible to body or proximity effects which can significantly decrease the accuracy of the touch localization when used with thick dielectrics. Proximity effects cause the sensor array to be prone to "false" or otherwise inaccurate touch signals or sensor signals. For example, a large conductive body proximate the sensor array may cause the capacitive touch sensor to generate a touch signal (a signal indicating that the array has been touched) when a hand or other object nears the sensor array. The capacitive nature of the large conductive body affects the capacitive loading of the sensor array and may even appear as a touch to the capacitive touch sensor. Large conductive bodies may be hands, forearms, or other objects which can affect the capacitive sensing of the sensor array even though the object is not in contact with (up to several feet away from) the sensor array.
Proximity effects are also associated with a display. A high dielectric constant in the substrate results in capacitive loading from conductive bodies to the rear of the sensor array and also results in proximity effects. Heretofore, capacitive touch sensors have thin dielectrics such as a 0.001 inch or less layer of silicon dioxide. The thin face plate or layer is less affected by the proximity effects. However, thin face plates are prone to scratching and wear.
Conventional capacitive touch sensors are also disadvantageous if they are used with thick dielectrics because they are susceptible to electromagnetic noise from electronic components associated with the display or other devices external the sensor and subject to proximity effects. Capacitive sensors often employ a rear guard layer to prevent electric and magnetic interference generated by the display or other electrical components from affecting the measurement of capacitive loading. The rear guard layer is generally a transparent conductor which is placed on the rear surface of the sensor. Rear guard layers are expensive and often degrade the optical performance of the display, especially the performance of flat screen displays.
Thus, there is a need for a capacitive touch sensor having a sensor array which is easy to manufacture and low cost. Preferably, the sensor array has a reduced number of sensor bars. Also, there is a need for a capacitive touch sensor which does not require a rear guard layer and can be utilized with a screen or window having a thick face layer. Further, there is a need for a capacitive touch sensor which is less susceptible to proximity effects and external electromagnetic noise. Additionally, there is a need for a capacitive sensor topology which is usable with thick dielectrics and which exhibits stability.